The invention relates to a thermodynamic oscillator having at least one displacer which is displaceable in a working space filled with the working medium, at the resonance frequency of the oscillator. The displacer divides the working space into an expansion space and a compression space of different substantially constant temperatures, which communicate with each other via a regenerator, the movement of the displacer due to pressure fluctuations in the working medium being coupled to a piston or a further displacer, respectively, which is also displaceable in the working space. The working space is connected via at least one release valve and at least one supply valve to a reservoir which is filled with the same working medium as that of the working space and whose pressure lies between a maximum and a minimum working pressure of the working medium.
On page 270-273 of the book "Stirling Engines" of 1980 by G. Walker (ISBN 0-19-856209-8), a thermodynamic oscillator of the kind mentioned in the opening paragraph has been described. This known oscillator has a so-called central position control for the piston, whereby the consequence of working medium leaking between the compression space and a gas buffer space forming part of the working space and compensated for by means of connections between these spaces and reservoirs in which a pressure prevails which is comparatively low with respect to the average working pressure. One connection comprises two release valves in series arrangement for blowing off the compression space through a first reservoir to the gas buffer space for compensating leakage from the gas buffer space to the compression space. The other connection comprises two supply valves in series arrangement for supplementing working medium to the compression space through a second reservoir from the gas buffer space for compensating leakage from the compression space to the gas buffer space. Therefore, the original central position of the piston is maintained in the case of leakage bothe in one and in the other direction. G. Walker has given no information about the mechanical pre-stress of the release and supply valves. However, it has to be assumed that the valves are biased by only a comparatively low mechanical pre-stress if it is to be possible for a sufficient compensation for leakage to be obtained. At any rate, it is clear that a variation of the ambient temperature in the known oscillator does not offer compensation for the resulting variation of the average work pressure. As a result of this, the thermodynamic spring constant of the work-medium and hence the resonance frequency of the known oscillator varies with the varying ambient temperature. The resulting variation of the phase difference between the movements of the displacer and the piston leads to a varying efficiency which is not an optimum.